1. Field of the Invention
The present invention relates to an input and output means for computer system storage and a software execution method using same, for preventing a computer system being fatally damaged by a computer virus that becomes a problem when providing new software for trial.
2. Description of the Prior Art
Trial software and trial data are exchanged via various communication media, and there are cases in which computer systems are damaged by computer viruses that become attached accompanying trial software and trial data distributed via the Internet in particular. Such damage can alter an operating system (OS) and, if it continues after such an alteration, in most cases it becomes necessary to reinstall the OS on the computer system, canceling the intended computer processing.
Generally, to prevent such damage, trial software is tried out, or characteristics of files created by a virus produced by reading trial data or mail data are detected and the file removed, or files that have a computer virus are erased before trying out trial software or reading trial data or mail data. Because new viruses are distributed one after another, it is difficult to prevent damage at the start of distribution using computer virus removal software.
In the case of such damage produced by a computer virus, there are known computer systems that substantially eliminate this kind of damage caused by computer viruses by using a function provided to reset the computer system before trying out trial software, or reading trial data or mail data.
Connectix Corporation's Virtual PC, for example, uses a method in which trial software and the like is operated on a virtual system using another, second, disk image file of the initial disk image file. Here, during operation, Virtual PC performs processing using the second disk image file, but when shutting down Virtual PC, when required, a procedure is used to have the second disk image file record reflected in the initial disk image file. Therefore, if a computer virus causes damage during the operation of Virtual PC, it is possible to return to the initial state by discarding the second disk image file.
As shown by the block diagram of FIG. 1, with VMware, Inc.'s VMware, the guest OS does not directly access the hard disk, but performs reads and writes via a virtual disk provided by VMware. FIG. 1 shows a host application operating on a host OS, a virtual platform (VP) application operating on a host OS, and a guest OS application operating on a guest OS. Thus, an application-operating environment can have a plurality of modes. Also, hard-disk read/write operations are carried out via a VP application; the following three read/write modes are available.    (1) Persistent mode: This mode performs hard-disk reads and writes in real time, as in the case of an ordinary hard disk.    (2) Non-persistent mode: A history of changes is written not to the hard disk but to a redo log, and the redo log is discarded when the guest OS terminates, so ultimately the hard disk is not written to.    (3) Undoable mode
Similarly to the case as in the above-mentioned non-persistent mode, a change history is written to the redo log and the contents of changes are saved, and when the guest OS is being terminated it is possible to choose to commit the contents of the redo log to the hard disk, discard the redo log, or to keep the contents instead of discarding the redo log.
In this way, computer systems up to the present use a disk image file to carry out a processing and, if required, the disk image file has been stored on the hard disk of the computer system. The examples described above provide a mode in which there is no direct writing to the hard disk, but to realize this mode, intermediate code, intermediate data and other such interim data-processing states are saved on the hard disk as required. This saving to the hard disk is undesirable from the standpoint of security and makes it more possible to create a computer virus.
As is well known, a computer virus causes damage by rewriting system files stored on the hard disk and erasing or modifying data files. This being the case, a computer system that is saved on a hard disk, such as described above, is more susceptible to damage caused by a computer virus.
The present invention is proposed in view of the above situation, and has as one object to provide an input and output means for computer system storage that by having a configuration that can stop a hard-disk write capability when endeavoring to prevent damage caused by a computer virus, but can enable a hard-disk write capability in cases such as when large-scale data processing is performed, is able to prevent a computer virus from fatally damaging a computer system while minimizing any loss of convenience. Another object is to provide a software execution method using the above input and output means for computer system storage that, by using the input and output means for computer system storage, enables trial software, trial data and mail data to be safely tried.